If you have Keynote on a Mac, you can use almost any theme that's 1024x768. Just create a presentation on the Mac, make 1 slide for each master slide type, then upload the presentation to the iPad. You won't be able to use it as a Keynote for iOS native theme, but you'll have the theme pages.
If you don't have Keynote for Mac, there are also several "template" providers who sell in the app store. Just do a search on Keynote and you'll see them.